1. γ*p →Δ(1232) →πN Results from CLAS
Cole Smith
University of Virginia
Japan-US Workshop on
Electromagnetic Meson Production and Chiral Dynamics
Apr 2005 Osaka, Japan
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

2. Δ(1232) Program at JLAB γv e e’ N 3/2 1/2 -1/2 -3/2 A1/2 S1/2 A3/2
Physics Goals
Helicity amplitudes A3/2 A1/2 S1/2 vs. Q2
Deformation of N and Δ (E2/M1, C2/M1).
Compare with quark models, lattice QCD.
Reality
Resonant and non-resonant contributions require models to separate.
Theoretical tools: Effective Lagrangian (MAID,XPT), dynamical (Sato-Lee,DMT), Dispersion Relations (Aznauryan).
Experiment: Identify observables sensitive to Born and rescattering terms in models.
Experiments at JLAB
Hall A – High luminosity, recoil polarization.
Hall B – Large acceptance and kinematic coverage, polarized target and beam.
Hall C – High Q2, search for onset of pQCD scaling (E2/M1→ 1). e e’ γv N
△(1232)
M1, E2, C2
3/2
1/2
-1/2
-3/2
A1/2
S1/2
A3/2
Δ3/2+(1232)
N1/2+(938) λΔ λN
Helicity conserving
Spin flip
Double spin flip
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

3. e / e / e e What do E2/M1 and C2/M1 ratios measure?
E0+,S0+,M1-,S1- E1+,S1+,M1+,…
M1, E2, C2
Short Range Physics?
Gluon exchange → D-state admixtures
Deformation of N / Δ
Long Range Physics?
χSB → Light pion mass
Excite qq pairs from vacuum
Shape of pion cloud
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

4. γ*p→Δ(1232) Magnetic Dipole – Quenched Lattice vs. Dynamical
C. Alexandrou et al, PRL, 94, (2005)
Pion Cloud
Extrapolation of lattice GM* to Q2=0 is within 10% of experiment (where are the pions?).
Predicted form factor falls with Q2 more slowly than data.
Fits of dynamical pion models to π photoproduction data suggest 30-50% of M1 photocoupling strength near Q2=0 due to meson rescattering at EM vertex.
Lattice shows a decreasing GM* coupling strength as quark mass decreases.
Uncertainties: size of lattice volume (too small?), effects of unquenching (qq loops, Δ decay)
Sato,Lee PRC 63, (2001)
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

5. Quadrupole Transition – Lattice QCD vs. Pion Cloud Models
γ*p→Δ(1232)→π0p
C. Alexandrou et al, PRL, 94, (2005)
E2/M1 (%)
Quenched lattice consistent with E2/M1 data !
C2/M1 (%)
Low Q2 behavior of C2/M1 strong test of calculations !
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

6. Reaction Models – Need to better constrain non-resonant backgrounds
Non-resonant multipoles not well determined in models.
Yang, Kamalov nucl-th/ (DMT model)
Sato, Lee nucl-th/
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

7. CLAS e1e Run (Nov-Dec 2002) Trigger Threshold Elastic Fiducial Cut
Beam energy GeV
Q2= GeV2 W<1.4 GeV
Beam polarization ~70%
Current 10 nA
LH2 target thickness 2.0 cm
Q2 (GeV2)
Trigger Threshold
Elastic
Elastic and inelastic data taken simultaneously. Normalization error ~ 3-4%
Refined analysis since Aug 2004
More accurate target window background subtraction.
Proton energy loss in target accounts for target geometry
Elastic Bethe-Heitler background rejection uses finer W steps.
W (GeV)
Fiducial Cut
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

8. CEBAF Large Acceptance Spectrometer (CLAS)
Six identical sectors
5 T toroidal B-field
Δθ= degrees
Δφ = 0-50 degrees
Δp/p =
π0 electroproduction
Proton detected
Lorentz boost forms cone surrounding q vector.
Cone angle Q2,W dependent
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

9. Pion Electroproduction Structure Functions
M1+ dominance. Re(M1+)=0
d2σ(MAID) vs θ* and φ* at Δ(1232) peak.
Grid shows experimental (cosθ*,φ*) bins.
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

10. π0p Center-of-Mass Coverage - θLAB vs. φLAB of Detected Proton
For Q2 < 1 GeV
Torus coils block some angles, adjacent sectors provide additional coverage
For Q2 > 1 GeV
Full c.m. coverage is possible
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

11. Acceptance Limitations at Low Q2
Magnet coils create acceptance asymmetry in φ*. Potential source of systematic error in σLT extraction.
Mini-torus magnet coils
Beam pipe hole exists in all low Q2 experiments. Removes yield near φ*=π which lowers sensitivity to σLT.
Center-of-mass Acceptance (GEANT)
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

12. Acceptance Monte Carlo Simulations
Generated ~25M events using MAID03 physics model.
W: 15 bins ( GeV)
Q2: 6 bins ( GeV2)
Cos θ*: 10 bins
φ*: 24 bins
GEANT simulation includes:
Accurate magnet coil geometry
Proton rescattering from coils.
Drift chamber resolution.
Bethe-Heitler radiation
Hadronic interactions (FLUKA)
Comparison of Geant and Data φ*
cos θ*
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

13. Bethe-Heitler Background (elastic tail)
A potential source of φ* asymmetry which can bias extraction of σLT
Largest background near φ*(proton)= 180o with tails extending beyond.
Suppressed using overconstrained elastic kinematic cuts on proton angle.
Residual BH tails subtracted using MC simulations.
MM2 cut: < MM2 < 0.08 GeV2
After cut
Missing Mass2
Missing Mass2
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

14. Typical pπ0 Cross Sections Near Pion Threshold
Q2 = 0.2 GeV2 W= GeV
Preliminary
Preliminary
Preliminary
Preliminary φ*
cos θ*
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

15. Analysis: Obtain Legendre Coefficients
φ* Fits
Extract 3 structure functions for each cos θ* bin.
Fit cos θ* to obtain Legendre coefficients.
Works best with full angular distributions (no holes).
Global Fits
Fit φ*,cos θ* simultaneously
More constrained fit
Smooths over missing bins.
Both methods used to check systematics of extracted Legendre coefficients.
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

16. CLAS Results for W=1.10-1.16 GeV at Q2 = 0.2 GeV2
Structure functions from φ* fits
Legendre coefficients from global fits
Preliminary
Preliminary
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

17. Model Dependence in Legendre Coefficients: 1.10 < W < 1.16 GeV
Preliminary
E1+
S1+
Largest differences between dynamical models occur for A1 and D0.
Largest difference between MAID and dynamical models is for D1, which is largely sensitive to Re(S1+)
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

18. Q2 Dependence of Legendre Coefficients – D1
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

19. Q2 Dependence of Legendre Coefficients – D0
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

20. Q2 Dependence of Legendre Coefficients – D0’
Model sensitivity in D0/ maximized at W=1.22 GeV in σLT/ whereas for σLT it vanishes.
Joo, PRC 70, (2004)
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

21. Q2 Dependence of Legendre Coefficients – A0
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

22. Q2 Dependence of Legendre Coefficients – A1
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

23. Q2 Dependence of Legendre Coefficients – A2
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

24. Q2 Dependence of Legendre Coefficients – C0
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

25. Typical Cross Sections vs cos θ* and φ*
Q2 = 0.2 GeV2 W=1.22 GeV
Preliminary
Preliminary
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

26. W Dependence of Legendre Coefficients – A0,A1,A2,C0,D0,D1
Preliminary
(M1+ dominance)
Preliminary
Resonant Multipoles
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

27. Response Functions - Comparison to Bates at Q2=0.127
W=1.232
Mertz
Kunz
Sparveris
MAID2003
CLAS Preliminary E=1.046 Q2=0.16
DMT
Sato-Lee
W=1.17
Model variation negligible between Q2=0.127 and 0.16.
Good agreement between Bates and CLAS for T+L and TT measurements.
Disagreement with Kunz point (■) for LT.
Global fit (-----) favors Sato-Lee model at W=1.232.
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

28. Is the Siegert Theorem Relevant?
Many models use the long wavelength limit q*=0 to constrain S1+ as Q2→0:
Long wavelength limit
Data so far do not satisfy this condition. Models show large variation.
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

29. Summary and Future
Nearly complete angular distributions obtained for pizero electroproduction for Q2 = GeV2 and W = GeV.
W and Q2 dependence of Legendre coefficients extracted from fits to φ* and cos θ* distributions.
Both previous and current preliminary CLAS data show clear disagreement with MAID in D0 and D1 (σLT) below the Δ(1232) resonance.
Preliminary E1+/M1+ in good agreement with MAID, while S1+/M1+ continues to show strong Q2 dependence.
Analysis of π+ and polarization data at low Q2 underway. Systematics analysis underway.
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

30. Backup Slides
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

31. Structure Functions vs Invariant Mass W
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

32. Structure Functions vs cosθ*
Preliminary
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

33. Δ(1232) – Global Fit to CLAS pπ0 + nπ+ data at Q2=0.4 and 0.65 GeV2
K. Joo et al., PRC 70, (2004)
H. Egiyan et al., to be submitted to PRC
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan

34. Δ(1232) Multipoles – JANR Global Fit Results
I.G. Aznauryan et al., PRC, (2005).
Cole Smith Japan-U.S Workshop on Electromagnetic Meson Production and Chiral Dynamics Apr 2005 Osaka, Japan